Method and system for determining a gap between a vibrational body and fixed point

ABSTRACT

A system for applying ultrasonic energy to a workpiece, the system including a horn stack; a mounting system upon which the horn stack is mounted; a source of energy coupled to the horn stack; an anvil having a surface for supporting the workpiece; and a controller configured to receive a resonant frequency of the horn stack, and to determine a quantity standing in known relation to a change in gap between the horn stack and the anvil.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/321,677, filed Dec. 29, 2005, now allowed, which application is anational stage filing under 35 U.S.C. 371 of PCT/US2005/047544, filedDec. 29, 2005, which claims priority to Provisional Application No.60/640,978, filed Jan. 3, 2005 and the disclosures of which areincorporated by reference in their entirety

FIELD OF THE INVENTION

The present invention relates to a method and system for determining agap between a vibrational body and a fixed point, and more particularlyto a system and method arriving at such a determination based upon theresonant frequency of the vibrational body.

BACKGROUND

In ultrasonic welding (sometimes referred to as “acoustic welding” or“sonic welding”), two parts to be joined (typically thermoplastic parts)are placed proximate a tool called an ultrasonic “horn” for deliveringvibratory energy. These parts (or “workpieces”) are constrained betweenthe horn and an anvil. Oftentimes, the horn is positioned verticallyabove the workpiece and the anvil. The horn vibrates, typically at20,000 Hz to 40,000 Hz, transferring energy, typically in the form offrictional heat, under pressure, to the parts. Due to the frictionalheat and pressure, a portion of at least one of the parts softens or ismelted, thus joining the parts.

During the welding process, an alternating current (AC) signal issupplied to a horn stack, which includes a converter, booster, and horn.The converter (also referred to as a “transducer”) receives the ACsignal and responds thereto by compressing and expanding at a frequencyequal to that of the AC signal. Therefore, acoustic waves travel throughthe converter to the booster. As the acoustic wavefront propagatesthrough the booster, it is amplified, and is received by the horn.Finally, the wavefront propagates through the horn, and is imparted uponthe workpieces, thereby welding them together, as previously described.

Another type of ultrasonic welding is “continuous ultrasonic welding”.This type of ultrasonic welding is typically used for sealing fabricsand films, or other “web” workpieces, which can be fed through thewelding apparatus in a generally continuous manner. In continuouswelding, the ultrasonic horn is typically stationary and the part to bewelded is moved beneath it. One type of continuous ultrasonic weldinguses a rotationally fixed bar horn and a rotating anvil. The workpieceis fed between the bar horn and the anvil. The horn typically extendslongitudinally towards the workpiece and the vibrations travel axiallyalong the horn into the workpiece. In another type of continuousultrasonic welding, the horn is a rotary type, which is cylindrical androtates about a longitudinal axis. The input vibration is in the axialdirection of the horn and the output vibration is in the radialdirection of the horn. The horn is placed close to an anvil, whichtypically is also able to rotate so that the workpiece to be weldedpasses between the cylindrical surfaces at a linear velocity, whichsubstantially equals the tangential velocity of the cylindricalsurfaces. This type of ultrasonic welding system is described in U.S.Pat. No. 5,976,316, incorporated by reference in its entirety herein.

In each of the above-described ultrasonic welding techniques, theworkpieces to be joined are disposed between the horn and the anvil,during the welding process. One way to weld is by fixing a gap betweenthe horn and the anvil. The gap between the horn and anvil creates apinching force that holds the workpieces in place while they are beingjoined. For the sake of yielding a uniform and reliable weldingoperation, it is desirable to maintain a constant gap between the hornand the anvil.

During operation, one or more components of the horn stack, includingthe horn, itself, generally experience an elevation in temperature.Thus, the horn stack generally experiences thermal expansion. As thehorn stack expands, the gap between the horn and the anvil isdecreased—a result inimical to the aforementioned goal of yielding auniform and reliable welding operation.

As the foregoing suggests, presently existing ultrasonic welding schemesexhibit a shortcoming, in that the gap between the horn stack and theanvil grows narrower, during successive welding operations.

SUMMARY OF THE INVENTION

Against this backdrop, the present invention was developed. According toone embodiment, a system is provided for applying ultrasonic energy to aworkpiece, the system includes a horn stack and a mounting system uponwhich the horn stack is mounted. The system also includes a source ofenergy coupled to the horn stack and an anvil having a surface forsupporting the workpiece. A controller is configured to receive aresonant frequency of the horn stack, and to determine a quantitystanding in known relation to a change in gap between the horn stack andthe anvil.

According to another embodiment, a system for applying ultrasonic energyto a workpiece includes a horn stack and a mounting system upon whichthe horn stack is mounted. The system further includes a source ofenergy coupled to the horn stack and an anvil having a surface forsupporting the workpiece. The system also includes a means fordetermining a quantity standing in known relation to a change in the gapbetween the horn

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an embodiment of a simple ultrasonic welding horn stackcoupled to an energy source.

FIG. 2 depicts an embodiment of a mounting system coupled to theultrasonic welding horn stack of FIG. 1.

FIG. 3 depicts an embodiment of a system for determining a length of agap between a horn and an anvil.

FIG. 4A depicts an exemplary embodiment of a table that may be used as apart of a gap-determining unit.

FIG. 4B depicts an exemplary embodiment of a method of determining a gaplength.

FIG. 5A depicts an embodiment of a simple rotary ultrasonic welding hornfor use in a continuous ultrasonic welding operation.

FIG. 5B depicts an exemplary embodiment of a method of determining a gaplength.

FIG. 6 depicts an exemplary embodiment of a system for maintaining asubstantially constant gap between a welding horn and an anvil.

FIG. 7 depicts an exemplary embodiment of a system for adjusting a gapbetween a horn and an anvil in an ultrasonic welding system.

FIG. 8A depicts an exemplary embodiment of a system for maintaining asubstantially constant gap between a horn and an anvil in an ultrasonicwelding system.

FIG. 8B depicts another exemplary embodiment of a system for maintaininga substantially constant gap between a horn and an anvil in anultrasonic welding system.

FIG. 9A depicts an exemplary embodiment of a force-determining unit.

FIG. 9B depicts another exemplary embodiment of a force-determiningunit.

FIG. 10 depicts an exemplary embodiment of a system for adjusting a gapbetween a horn and an anvil in an ultrasonic welding system.

FIG. 11A depicts the surface of a horn driven by an acoustic signalpropagating along the longitudinal axis of the horn.

FIG. 11B depicts the surface of a horn driven by an acoustic signal ofsmaller magnitude than that of FIG. 11A, as that signal propagates alongthe longitudinal axis of the horn.

FIG. 12A depicts an exemplary embodiment of a system for controlling thegap between a horn and an anvil.

FIG. 12B depicts another exemplary embodiment of a system forcontrolling the gap between a horn and an anvil.

FIG. 13 depicts an exemplary embodiment of a method for combining theoperations of an adjustor and an amplitude determination module.

FIG. 14 depicts another exemplary embodiment of a method for combiningthe operations of an adjustor and an amplitude determination module.

DETAILED DESCRIPTION

Various embodiments of the present invention will be described in detailwith reference to the drawings, wherein like reference numeralsrepresent like parts and assemblies throughout the several views.Reference to various embodiments does not limit the scope of theinvention, which is limited only by the scope of the claims attachedhereto. Additionally, any examples set forth in this specification arenot intended to be limiting and merely set forth some of the manypossible embodiments for the claimed invention.

FIG. 1 depicts an example of a simple horn stack 100 that is coupled toan AC source of electrical energy 102. As can be seen from FIG. 1, thehorn stack 100 includes a converter 104, a booster 106, and anultrasonic welding horn 108. During operation, the AC source supplieselectrical energy to the converter 104, which responds thereto bycompressing and expanding at a frequency equal to that of the AC signal.Therefore, acoustic waves travel through the converter 104 to thebooster 106. As the acoustic wavefront propagates through the booster106, it is amplified, and is received by the welding horn 108. (In someembodiments, the horn 108 is designed to achieve a gain, eliminating theneed for a booster 106.) Finally, the wavefront propagates through thehorn 108, whereupon it is imparted to workpieces (not depicted inFIG. 1) that are positioned between the welding horn 108 and an anvil110. Other examples of horn stacks are known in the art, and functionwith the following systems, schemes, and methods disclosed herein.

The horn 108 is separated from the anvil 110 by a distance labeled “Gap”in FIG. 1. The process of imparting frictional energy to the workpiecescauses the various elements of the horn stack 100 to elevate intemperature. As the elements of the horn stack 100 elevate intemperature, they exhibit thermal expansion, meaning that the gapbetween the horn 108 and the anvil 110 is likely to change in dimension,depending upon the particular manner in which the horn stack 100 ismounted.

FIG. 2 depicts a simplified exemplary mounting scheme for the horn stack100 of FIG. 1. The mounting scheme makes use of a rigid, generallytripartite, frame 200. The frame 200 includes a first portion 202 uponwhich the anvil 110 is mounted, and a second portion 206 that isadjoined to a nodal point on the horn stack 100. For example, the secondportion 206 of the frame is depicted in FIG. 2 as being coupled to themidpoint 208 of the booster 106. A third portion 204 of the frame 200extends between the first and second portions 202 and 206.

The mounting system 200 maintains a substantially fixed distance betweena workpiece-supporting surface 210 of the anvil 110 and a portion of thehorn stack 100. In this case, the mounting system 200 maintains asubstantially fixed distance between the upper surface 210 of the anvil110 and the midpoint/nodal point 208 of the booster 106. Therefore,should the horn stack 100 expand during operation, the horn stack 100expands outwardly from the midpoint 208 of the booster 106, along thelongitudinal axis of the stack 100, as indicated by the arrows labeled“Expansion” in FIG. 2. It is understood that a variety of other mountingsystems may also maintain a substantially fixed distance between theupper surface 210 of the anvil 110 and a portion of the horn stack 100,and such other mounting systems are within the scope of the presentapplication.

Given the mounting arrangement of FIG. 2, thermal expansion of theconverter 104 and upper portion of the booster 106 produces no effect onthe gap length (because of the position of these elements relative tothe point 208 at which the frame 200 joins the stack 100, these elementsare free to expand upwardly, i.e., away from the anvil 110). On theother hand, the gap length is affected by expansion of the lower portionof the booster 106 and by expansion of the horn 108—as these elementsexpand, they expand toward the anvil 110, and the gap contracts.

According to one embodiment, the converter 104 and booster 106 aremaintained at a substantially constant temperature. For example, theconverter 104 and booster 106 may be cooled by a cooling system, such asby one or more fans that circulate relatively cool air to the surfacesof the converter 104 and booster 106, so as to substantially maintaintheir temperatures, and to thereby substantially suppress their thermalexpansion. Therefore, according to such an embodiment, any change inlength of the horn stack 100 may be considered as being substantiallydue to expansion of the welding horn 108.

Furthermore, according to some embodiments, the horn 108 is cooled by acooling system, so as to suppress or reduce its propensity to heat upduring operation. Generally, such a scheme does not totally eliminatethermal expansion of the horn 108, meaning that it still exhibits somedegree of thermal expansion, which should be accounted for, if the gaplength is to be maintained substantially constant.

It is known that the length of a given body is inversely proportional tothe given body's resonant frequency. Stated another way, as a body growsin length, it exhibits a lower resonant frequency. Therefore, as thehorn stack 100 grows in length, as occurs, for example, by virtue ofthermal expansion, it exhibits a lower resonant frequency. Specifically,the length of a body, l, is related to its resonant frequency, f, by thefollowing equation:

${l \approx \frac{\sqrt{E/\rho}}{2f}},$

where E represents the modulus of elasticity of the object, and where ρrepresents the density of the object. If the object is compound (e.g.,is made up of multiple parts or has various sections made from differentmaterials, etc.), E and ρ may be assigned values representing thebehavior of the materials, considering its various parts (e.g., may be aweighted average, etc.).

According to some embodiments, the energy source 102 detects theresonant frequency, f, of the horn stack 100, in order to generate an ACsignal equal in frequency thereto. For example, the energy source 102may deliver a sinusiodal signal exhibiting a particular peak-to-peakvoltage (or root-mean-square voltage) to the horn stack 100. Whilekeeping the peak-to-peak (or RMS) voltage of the sinusoidal signalconstant, the energy source 102 adjusts the frequency of the signal, andseeks out the frequency at which the least current is drawn by the hornstack 100—this frequency is the resonant frequency of the horn stack100. Accordingly, per such embodiments, the resonant frequency of thestack 100 may be obtained from the energy source 102. According to otherembodiments, the resonant frequency of the stack 100 may be detected byobservation of the stack 100 with a detector.

Upon obtaining the resonant frequency of the horn stack 100, the overalllength of the stack 100 may be obtained by relating, in a manner similarto the aforementioned physical principles, resonant frequency to hornstack length. Given that the converter 104 and booster 106 are cooled,so as to substantially suppress the effects of thermal expansionthereupon, the length of the horn stack 100 can be related to the gaplength. For example, according to the scheme of FIG. 2, the gap lengthand the length of the horn 108, l, are related by the followingequation:

gap length≈D−l,

where D is an approximately constant value that represents the lengthbetween the top of the horn 108 and the workpiece-supporting surface 210of the anvil 110.

FIG. 3 depicts a system for determining the length of the gap between awelding horn 108 and the workpiece-supporting surface 210 of the anvil110. The system of FIG. 3 includes an ultrasonic power supply 300 (e.g.,an electrical power supply that delivers an AC signal to converter,which, in turn, transduces the signal into an acoustic wave) thatdelivers an acoustic signal to a horn (and booster) 302. The ultrasonicpower supply 300 is controlled by a controller circuit, such as by aprocessor in data communication with a memory device that storesfirmware/software controlling the operation of the ultrasonic powersupply 300. Alternatively, the controller circuit may be embodied as ahardware-based control loop. In either event, the controller of theultrasonic power supply 300 identifies the resonant frequency of thehorn stack, and commands power supply signal generation circuitrytherein to cooperate with the converter to yield an acoustic signalequal in frequency thereto. The controller within the power supply 300may interface to a gap-determining unit 304.

The gap-determining unit 304 receives the resonant frequency of the hornstack, and generates a quantity standing in known relation to the gaplength. According to one embodiment, the gap-determining unit 304 is asoftware module executing upon a processor coupled to a memory unit. Thegap-determining unit 304 may execute upon the same processor upon whichthe firmware controlling the ultrasonic power supply 300 executes.Alternatively, it may execute upon a different processor that is in datacommunication therewith. In either event, the software/firmware executedby the gap-determining unit 304 may function according to the schemes(below) discussed with reference to FIGS. 4A-5B.

According to an alternative embodiment, the gap-determining unit 304 mayreceive the resonant frequency of the horn stack from a source otherthan the ultrasonic power supply 300. For example, the system mayinclude a detector 306 that observes the horn stack, measures theresonant frequency thereof, and communicates the resonant frequency tothe gap-determining unit 304. In the discussion that follows, it isassumed that the resonant frequency originates from the ultrasonic powersupply 300, for the sake of example only.

FIG. 4A depicts a scheme by which the gap-determining unit 304 mayoperate. The gap-determining unit 304 may include a table 400 stored ina memory device. The table 400 is organized according to resonantfrequency, and relates a gap length G to a resonant frequency, f. Thus,upon receiving a resonant frequency, f, the gap-determining unit 304uses the resonant frequency to access the table 400, and to determine agap length G corresponding to the resonant frequency, f. For example,assuming that the gap-determining unit 304 receives a frequency of f₂ asan input, the unit 304 responds by accessing the table 400 to identify arow corresponding to frequency f₂. Upon identification of the row, thegap length entered therein, G₂, is returned. Optionally, the table 400may be accessed to determine the length of the horn stack 100, L, or todetermine any other quantity standing in known relation to the gaplength. Assuming that the gap-determining unit 304 receives a valuef_(x) as an input, and assuming that f_(x) falls between successivetable entries (i.e., f_(i)≦f_(x)≦f_(i+1)), then the gap-determining unit304 may access the table 400 to obtain gap length values G_(i) andG_(i+1), and may interpolate between the two values to arrive at a gaplength corresponding to the resonant frequency, f_(x).

The various entries in the table 400 may be populated ex ante by aheuristic process, in which the length of the horn stack 100 and thelength of the gap are recorded for each frequency, f, within the table400. Alternatively, the various entries in the table 400 may bepopulated by theoretical calculation, in a manner similar to thatdescribed above.

FIG. 4B depicts another scheme by which the gap-determining unit 304 mayoperate, theoretical computation. For example, the gap-determining unit304 may begin its operation by receiving the resonant frequency of thehorn stack 100, f, as shown in operation 402. Thereafter, the unit 304responds by calculating the length of the horn 108, L, based upon theresonant frequency, such as by use of an equation based upon thephysical principles underlying the equation shown in operation 404.Finally, as shown in operation 406, the unit 304 may relate the length,L, determined in operation 404, to a gap length, based upon knowledge ofthe particular geometric constraints arising from the mounting schemeemployed. For example, in the context of the mounting scheme of FIG. 2,the gap length may be found as:

Gap Length=D−L,

where D represents the distance between the top of the horn 108 and theworkpiece-supporting surface 210 of the anvil 110, and L represents thelength of the horn.

FIG. 5A depicts an example of a welding horn 500 that is used forcontinuous ultrasonic welding. The horn 500 therein includes alongitudinal axis 502 about which the horn 500 may rotate. The horn 500is constrained by a mounting system (not depicted in FIG. 5A), so that agap is maintained between the horn and the anvil 504. The horn stack maybe mounted at any nodal point on the system. The longitudinal axis 502of the horn is substantially parallel to the workpiece-supportingsurface 506 of the anvil 504.

The aforementioned principle of determining the length of the gapbetween a horn and an anvil based upon the resonant frequency of thehorn stack is applicable to the horn 500 of FIG. 5. As materials expandthermally, they do so in equal proportions in all directions. Therefore,the following technique, depicted in FIG. 5B, may be used to determinethe length of the gap between the horn and the anvil.

Initially, as shown in operation 508, the resonant frequency of the hornstack is received. Thereafter, the length of the horn 502, L, isdetermined based upon the frequency, in like manner as described above(operation 510). As before, the horn stack of FIG. 5A is cooled so thatthe converter (not depicted in FIG. 5A) and booster (not depicted inFIG. 5A) remain at substantially constant temperatures during operation,thereby suppressing their thermal expansion and the effects on thesystem resonant frequency.

Since the horn 500 expands proportionally in all dimensions, the ratiobetween its length, L, and its radius, B, remains constant. Therefore,after calculation of the length of the horn 502, its radius may bearrived at by multiplication of the length by the aforementioned ratio,B, as shown in operation 512. Finally, the length of the gap may bedetermined by subtracting the radius from the distance, D, between thelongitudinal axis of the horn 500 and workpiece-supporting surface 506of the anvil 504, as shown in operation 514.

It should be noted that the results of the method described with respectto FIG. 5B may be stored within a table, as described with reference toFIG. 4A. Thus, the gap length, or any value standing in known relationthereto, may be obtained by virtue of accessing such a table, based uponthe resonant frequency of the horn stack.

FIG. 6 depicts a control system for maintaining a substantially constantgap between a horn and an anvil, based upon observation of the resonantfrequency of the horn stack. The system includes a horn stack 600 and apower supply 602 coupled thereto. According to one embodiment, the powersupply 602 determines the resonant frequency of the horn stack 600, asdescribed above.

Coupled to the horn stack is a position adjustor 606. The positionadjustor 606 adjusts the horn stack 600, either toward or away from theanvil, under the control of an input signal. A known relationship existsbetween the input signal delivered to the adjustor 606 and its responsethereto. The position adjustor 606 is in data communication with acontrol signal generator 604. The control signal generator 604 receivesthe resonant frequency of the horn stack as an input, and generates acontrol signal that is delivered to the position adjustor 606. Thecontrol signal generator 604 yields a control signal that maintains asubstantially constant gap between the anvil and the horn, given theresonant frequency of the horn stack 600 and the relationship betweenthe response of the position adjustor 606 and its input signal.

The control signal generator 604 may be embodied as a controllercircuit, such as a processor in data communication with a memory devicethat stores firmware/software in accordance with the aforementionedprinciples. It may alternatively be embodied as an ASIC yielding theaforementioned control signal so as to maintain a substantially constantgap. In the following portion of the disclosure, a particular embodimentof a position adjustor is disclosed. It is not necessary to use theposition adjustor disclosed below for practice of the invention. Also,the preceding portion of the specification was directed towardparticular methods of determining the length of a horn or the length ofa gap, based upon the resonant frequency of the horn stack. According toother embodiments, such determinations may be arrived at by measurementof the temperature of the horn stack, or of its various components.

FIG. 7 depicts an exemplary embodiment of a system for adjusting the gapbetween a horn and an anvil. The system therein includes a horn 700oriented above a workpiece-supporting surface 702 of an anvil 704. Thehorn 700 is rigidly coupled to a frame 706. The frame 706 includes aslide 708 that engages a receiver 710, so that the frame 706 and horn700 may translate vertically.

The frame 706 also includes a force-receiving plate 712 that is coupledto the frame 706 by a pair of members 714. A force is applied to theforce-receiving plate 712 by a force applicator (not depicted in FIG.7). The force urges the horn 700 toward the anvil 704. The direction ofthe force is indicated by the arrow 713. The force has the effect ofcausing a contact surface 716 to abut an elastic deformable stop 718.The force exerted upon the elastic deformable stop 718 causes the stop718 to deform, and to thereby exhibit a downward deflection (i.e., adeflection in the direction of the anvil 704). Generally, the greaterthe force applied to the plate 712, the greater the downward deflectionexhibited by the stop 718. The greater the deflection exhibited by thestop 718, the smaller the gap between the horn 700 and the anvil 704.

To maintain a constant gap between the horn 700 and the anvil 704, thefollowing scheme may be employed. While the horn 700 is at itsunelevated temperature, an initial force is applied to the plate 712, tocause the gap between the horn 700 and the anvil 704 to be establishedat an “ideal” length. As the horn 700 thermally expands duringoperation, the gap grows smaller. To counteract this effect, the forceapplied to the plate 712 is reduced, causing the stop 718 to exhibit alesser deflection, meaning that the horn 700 and frame are translatedupwardly (i.e., away from the anvil). Thus, the gap between the horn 700and the anvil 704 may be maintained substantially constant by controlledapplication of force to the plate 712. To ensure the functionality ofthis scheme the initial force applied to the plate 712 should be ofsufficient magnitude to cause the stop 718 to exhibit a deflection atleast as great in extent as the expected thermal expansion to becounteracted.

The deformable stop 714 is elastic, and preferably has a relatively highmodulus of elasticity. By selection of a material having a relativelyhigh modulus of elasticity, a circumstance is set up in which the forcerequired to deflect the stop 714 is relatively great compared to theprocess force (i.e., the force exerted by the horn on the workpiece).Such an arrangement provides for ease of control design. According toone embodiment, the stop 714 may be made of steel, or another suitablematerial. According to one embodiment, the force exerted upon the stop714 does not cause the material therein to exit its elastic range (i.e.,the stop 714 will return to its original shape upon withdrawal of theforce). Further, according to one embodiment, the stop 714 exhibits adeflection that is proportional to the force applied thereto, i.e.,there exists a linear relationship between the force applied to the stop714 and the extent of deflection exhibited thereby.

FIG. 8A depicts an example of a control system for use with theexemplary adjustment system of FIG. 7. (The various units 804-810 ofFIG. 8A, discussed below, may be embodied as software modules stored ina computer-readable medium and executed by a processor, or may beembodied as dedicated hardware, such as one or more application-specificintegrated circuits, or as a field-programmable gate array. Further, theunits 804-810 may be combined or divided as a matter of design choice.)As can be seen from FIG. 8A, the system includes a horn 800 that iscoupled to a source of ultrasonic power 802. A gap-determining unit 804determines the gap between the horn 800 and an anvil (not depicted inFIG. 8). According to one embodiment, the gap-determining unit 804obtains the resonant frequency of the horn stack from the power source802, and determines the gap therefrom. According to another embodiment,the gap-determining unit 804 detects the resonant frequency of the horn800 by observation thereof. According to yet another embodiment, the gapdetermining unit 804 arrives at the gap length by measurement of thetemperature of the horn, inferring horn length therefrom, and arrivingat the gap length on the basis of the horn length.

The gap length arrived at by the gap-determining unit is supplied to aforce-determining unit 806. The force-determining unit 806 determinesthe force to be exerted upon the frame (e.g., plate 712 in FIG. 7), inorder to maintain the gap at a substantially constant length. The forcearrived at by the gap-determining unit 806 is supplied to a controlsignal generator 808. The control signal generator 808 develops acontrol signal, and communicates that control signal to a forceapplicator 810. The force applicator 810 exhibits a known relationshipbetween the received control signal and the force it exerts. Thus, thecontrol signal generator 808 develops the control signal in light ofthat relationship.

FIG. 8B depicts exemplary embodiments of the gap determining unit 804and force determining unit 806. (As was the case with the units of FIG.8A, the various units of FIG. 8B, discussed below, may be embodied assoftware modules stored in a computer-readable medium and executed by aprocessor, or may be embodied as dedicated hardware, such as one or moreapplication-specific integrated circuits, or as a field-programmablegate array. Further, the units of FIG. 8B may be combined or divided asa matter of design choice.) As can be seen from FIG. 8B, the gapdetermining unit 804 includes a length determining unit 812 and a gapfinding unit 814. The length determining unit 812 receives the resonantfrequency of the horn stack, and applies one of the methods describedwith reference to FIGS. 4A and 4B to find the length of the horn.Thereafter, the length of the horn is received by the gap finding unit814. The gap finding unit 814 arrives at the gap length, by virtue ofknowledge of the length of the horn and the particular geometry imposedby the mounting scheme (e.g., the gap length may be equal to thedifference between the length from the top of the horn to theworkpiece-supporting surface and the horn length, Gap=D−L).

After arrival at the gap length, this value is provided to theforce-determining unit 806. The force-determining unit 806 arrives atthe force to be applied to the frame, in order to keep the gapsubstantially constant. The force arrived at is a function of, amongstother things, the length of the stop, L_(stop), the modulus ofelasticity of the stop, E, the cross-sectional area of the stop, A, thedifference between the initial gap length and the gap length as arrivedat by the gap determining unit 804, Δ, and the assembled systemdeflection.

FIG. 9A depicts a scheme by which the force-determining unit 806 mayoperate. The force-determining unit 806 may include a table 900 storedin a memory device. The table 900 is organized according to resonant gaplength, G, and relates a force F to a gap length, G. Thus, uponreceiving a gap length, G, the force-determining unit 806 uses the gaplength to access the table 900, and to determine a force F correspondingto the gap length, G. For example, assuming that the force-determiningunit 806 receives a gap length of G₂ as an input, the unit 806 respondsby accessing the table 900 to identify a row corresponding to gap lengthG₂. Upon identification of the row, the force entered therein, F₂, isreturned. Optionally, the table 900 may be accessed to determine thecontrol signal, C, to be delivered to the force applicator 810, or todetermine any other quantity standing in known relation to the force tobe exerted on the frame. Assuming that the force-determining unit 806receives a value G_(x) as an input, and assuming that G_(x) fallsbetween successive table entries (i.e., G_(i)<G_(x)<G_(i+1)), then theforce-determining unit 806 may access the table 900 to obtain forcevalues F_(i) and F_(i+1), and may interpolate between the two values toarrive at a force corresponding to the gap length, G_(x).

The various entries in the table 900 may be populated ex ante by aheuristic process, in which the force to be applied to the frame and thecontrol signal corresponding thereto are experimentally determined foreach gap length, G, within the table 900. Alternatively, the variousentries in the table 900 may be populated by theoretical calculation, ina manner similar to that described below with reference to FIG. 9B.

FIG. 9B depicts another scheme by which the force-determining unit 806may operate, theoretical computation. For example, the force-determiningunit 806 may begin its operation by receiving the gap length calculatedby the gap determining unit 804, CG, as shown in operation 902.Thereafter, the unit 806 responds by calculating the difference betweenthe initial gap, IG, and the calculated gap, CG, as shown in operation904. This difference, Δ, refers to the amount by which the deflection ofthe stop must be reduced in order to return the gap to its initiallength. Thus, in operation 906, the new force to be applied to theframe, F_(new), may be arrived at by solving for F_(new) in the equationshown therein.

FIG. 10 depicts another exemplary embodiment of a system for adjustingthe gap between a horn and an anvil. Welding system 1010 has a weldingsystem 1030 fixed to support surface 1017 and an anvil 1021 fixed tosupport surface 1018. Welding system 1030 includes horn 1032, which issupported by horn support 1020 and is moveable in relation to surface1017, a fixed stop 1055 having support plate 1056, which are fixed inrelation to surface 1017, and an expandable pneumatic bladder 1061.

Bladder 1061 is used to apply the force to move horn support 1020 andhorn 1032 toward anvil 1021; the force is controlled by adjusting theair pressure in bladder 1061. As surface 1025 contacts fixed stop 1055,support plate 1056 deflects slightly under the applied force.

In one specific example, the minimum allowable force to weld a desiredproduct is 600 pounds (about 272 kg), which is generated by 30-psig(about 207 kPa) air pressure in bladder 161. The desired fixed gap is0.0020 inch (about 0.05 mm).

In operation with a titanium horn, it was determined that thetemperature will increase from room temperature by a maximum of 50° F.(about 27.7° C.), which will increase the horn length by 0.0010 inch(about 0.025 mm). As a result, the gap between horn 132 and anvil 121 isreduced to 0.0010 inch (about 0.025 mm), if no compensation is made. Thedeflection of support plate 156 is known to be 0.0010 inch (about 0.025mm) per 675 pounds force (about 306 kg-force). Therefore, the appliedforce with a room temperature horn must be at least 1125 pounds (about510 kg), or 60 psig (about 414 kPa). As the horn operates and increasesin length, the applied air pressure is reduced from 60 psig (about 414kPa) to 30 psig (about 207 kPa) to keep the gap between horn and anvilconstant.

A welding apparatus, generally configured to control the distancebetween the anvil and the horn by utilizing a deformable stop assembly,includes an anvil with a fixed stop, a horn, and a force applicatormounted so as to be able to apply force to press the horn against thefixed stop such that elastic deformation of the fixed stop provides finecontrol over the gap between the horn and the anvil. The apparatus mayinclude a sensing system to monitor a specific property of the horn andcontrol the force applied to the horn so as to hold the gap between thehorn and the anvil at a fixed value despite changes in the specificproperty. The property monitored could be, for example, temperature,length, or vibration frequency of the horn.

The use of a deformable, yet fixed stop to compensate for the hornlength increase, due to thermal expansion, can be used with a rotaryanvil, stationary anvil, rotary horn, stationary horn, or anycombination thereof.

In use, the workpieces to be joined would be positioned between the hornand the anvil, energy would be applied to the horn and the horn would beenergized, and a force would be applied to the horn to urge the hornagainst the fixed stop such that elastic deformation of the fixed stopprovides fine control of the gap between the horn and the anvil.

To employ the methods discussed above, one can determine data for asystem, and then fit it into equations that can be used in the controlsystem for a particular unit. Applicants have used the following methodfor the system described above, but this method can be applied to othersystems of different configurations. The equations were can be derivedusing engineering principles or using measured data from an individualsystem.

Equations 2-5 were best fits to linear systems of the two variables. Theslope and intercept of the equations were determined empirically frombest fitting measured data of the system. Measuring the relationshipbetween the variables can similarly yield the slope and intercept of anyparticular system. It is preferred that the systems behave linearly inthe operating regions, but if the systems are non-linear, a second orderor higher equation can be used.

Applicants have developed and used the method described following forcontrol of a gap during ultrasonic welding.

First, for a rotary ultrasonic system as described above, the followingparameters were determined.

(1) Horn diameter=6.88″

(2) Ambient temp. ° F.=65° F.

(3) Frequency at ambient temp.=19.986 KHz

(4) Pressure at which gap is set at =72.5 psig.

(5) Gap set point for the process=2 mils (1 mil=0.001 inch).

The material properties of the horn are also known,

(6) Coefficient of Thermal Expansion, α

α_(Titanium)=5.4×10⁻⁶ deg F./inch/inch

α_(Titanium)=5.4×10⁻⁵ deg F./inch/inch

When the system is energized and operating, the horn will increase intemperature. So next, one determines what would be the temperature,T_(final), at which there will be no gap left (i.e., 2.0 mil gap goes tozero, e.g., contact between horn and anvil) when welding continuously.This temperature is found by solving Equation 1:

$\begin{matrix}{T_{final} = \left( \frac{2*{IG}*10^{- 3}}{D*\alpha} \right)} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

In Equation 1, T_(final) is the temperature at which the Gap vanishes,IG is the initial gap (in mils) set and measured when the system isset-up and not in operation, D is the outer diameter of the rotary horn,and α is the coefficient of thermal expansion of the horn material.Solving Equation 1 using the above inputs for an aluminum horn gives atemperature of 172.7 deg F. where the gap will go to zero based onheating of the horn during operation. Thus, if the horn heats to 172.67°F., then there will be no gap left. Hence there is an upper bound fortemperature. The upper bound for any given system can be found usingequation 1 for a rotary system. One of ordinary skill in the art willalso appreciate that a similar equation can also be derived for othergeometries, and an upper operating temperature for avoiding a vanishinggap can be determined.

As it is difficult to measure the temperature on a dynamic resonatingstate of a horn, Applicants developed using a surrogate that gives anindirect, but accurate, measurement of temperature. Instead of directlymeasuring temperature, the frequency of the horn is determined bymeasuring the frequency of the horn during operation, and thendetermining temperature by using Equation 2 following:

λ_(min)=−0.0017*T _(final)+20.096  (Equation 2)

In Equation 2, λ_(min) is the minimum frequency at which the horn can beoperated before the gap goes to zero, and the coefficients of the linearequations have been determined empirically by experiment. SolvingEquation 2 for the input parameters, the gap will go to zero when thefrequency of the horn drops to less than 19,802 Hertz. Since thefrequency of the horn is a parameter than can be measured easily usingstandard equipment commonly used by those of ordinary skill in the art,one can determine using Equations 1 and 2 the minimum operatingfrequency of a rotary system that will keep the gap from closing, whichcan result in product damage and also damage the horn and/or anvil dueto the contact.

Using Equations 1 and 2, one now has the ability to relate gap totemperature and temperature to frequency. Hence, one can relate the gapto frequency. During normal operation, when the material is in the gap(or nip), it is difficult to measure the gap, but using the aboveprinciples, the frequency can be used to determine the gap. Therelationship between the frequency of the horn and the gap between thehorn and anvil can be determined using Equation 3 (which can be solvedfor either the gap as a function of frequency or vice versa) following:

λ=0.0965*Gap+19.7925  (Equation 3)

In Equation 3, λ is the horn frequency and the Gap is measured in mils(1 mil=0.001 inches). Solving Equation 3 for a gap of 1 mil gives afrequency of 19,889 Hertz. Note that there is now a way to determine thegap change as a function of frequency. Using the information thusdetermined by Equations 1-3, the force applied to the horn/anvilarrangement can be controlled to keep the operating gap constant as thetemperature and frequency of the horn change during operation of thewelding assembly.

To control the gap and keep it a constant operating value, the pressureapplied to the system is controlled, thereby compensating for thermalexpansion of the horn as it heats during operation. Referring back tothe example above, when the gap is reduced to 1 mil, one needs to reducethe pressure exerted on the system so that the system can keep or getback to original gap setting of 2 mils. Hence, to compensate for thethermal expansion, the pressure is reduced to get the gap to go back to2 mils.

To reduce the pressure properly, one first needs to determine therelationship between pressure and frequency, as shown by Equation 4following:

P _(compensation)=−367.3404*λ+7412.7731−P _(setpoint)  (Equation 4)

where P_(compensation) is the reduction in pressure (in pounds persquare inch P_(compensation) gage) on the system, λ is the frequencydetermined from Equation 3, and P_(setpoint) is the pressure at theinitial gap set point.

For example, using the above parameters, one can determine the pressurereduction needed to move restore an initial gap of 2 mils when the hornexpands 1 mil due to thermal expansion.

Example: What is the pressure compensation needed if the gap changed to1 mil?

First calculate the frequency for gap at 1 mil from Equation (3) (thatvalue is 19.889 KHz, as previously determined). Then substituting thevalues into Equation 4 yields,

$\begin{matrix}{P_{compensation} = {{{- 367.3404}(19.889)} + 7412.7731 - 72.5}} \\{= {106.7399 - 72.5}}\end{matrix}$

P_(compensation)=34.24 psig (reduction in operating pressure)

After the pressure has been determined, to compensate for thermalexpansion, it can be verified what is the gap at that pressurecompensation. This gap should be roughly equal to initial gap plus thegap change due to thermal expansion. To verify, first the relationshipbetween the Pressure and Gap is determined by Equation 5 following:

P _(Compensation)=35.461*(Gap @ PressureCompensation)+142.205  (Equation 5)

For example, at a pressure compensation of 34.24 psig (from Equation 4),one can rearrange Equation 5 and solve for the Gap:

Gap@Pressure Compensation=(34.24−142.205)/−35.461=3.045 mils

Thus, one can validate the model because the Initial Gap was set at 2.0mils and the gap change was 1 mil. Therefore, to compensate for a 1 milexpansion due to heating of the horn during operation, one would openthe gap by 1 mil, thereby restoring the original 2.0 mil gap.

Thus, using the equations (or deriving their equivalents for linearhorns or other geometries) discussed above for determining the operatingparameters, one can determine the operating limits for a rotaryultrasonic welding process. For example, the operating temperature limitis found using Equation 1 and value of Gap set point (target). Theoperating frequency limit of the ultrasonic horn is found using Equation2 and using the value of Temperature limit from Equation 1. Thefrequency at gap change is found using Equation 3 and using the value ofthe gap as input. The temperature at gap change is found using Equation2, but using the value of frequency determined from Equation 3. ThePressure Compensation for Gap change is found using Equation 4 but usingvalue of Frequency from Equation 3. The Gap at Pressure Compensation (atAmbient Temperature) is found using Equation 5, but using the value ofPressure Compensation from Equation 4.

There exists yet another scheme by which the gap between a horn and ananvil may be controlled. As mentioned previously, in the context ofultrasonic welding, a horn is driven by an acoustic signal, generally inthe realm of 20,000 to 40,000 Hz. FIG. 11A depicts the surface 1100 of ahorn, as an acoustic wave propagates along its longitudinal axis. Thedirection of propagation of the acoustic wave is depicted by the arrow1102. As can be seen from FIG. 11A, as an acoustic wave propagates alongthe longitudinal axis of the horn, the surface 1100 of the horn isperturbed, and exhibits a standing waveform 1104 thereupon. The standingwaveform 1104 exhibits a peak-to-peak amplitude, referred to as the“displacement” exhibited by the horn surface. The peak-to-peakamplitude, or surface displacement, is a function of the amplitude ofthe acoustic signal propagating along the horn. Of course, the amplitudeof the acoustic signal is a function of the amplitude of the electricalsignal supplied to the converter coupled to the horn. Thus, thedisplacement exhibited by the surface 1100 of the horn is a function ofthe amplitude of the electrical signal delivered to the converter.Typically, the greater the amplitude of the electrical signal deliveredto the converter, the greater the amplitude of the acoustic signalpropagating along the horn; the greater the amplitude of the acousticsignal, the greater the displacement exhibited on the surface 1100 ofthe horn.

As can be seen from FIG. 11A, the gap between the surface 1100 of thehorn and the surface of the anvil 1106 is a function of thedisplacement. As the horn exhibits greater surface displacement, the gapbetween the surface of the horn and the surface of the anvil diminishes.

Before proceeding further, it is pointed out that FIGS. 11A and 11B arenot drawn to scale, and that some features therein, such as the surfacedisplacement have been exaggerated for the sake of illustration. (Atypical horn may exhibit a surface displacement of approximately 2-3mils, when operating under normal conditions, for example.)

For the sake of discussion, the amplitude of the voltage signalstimulating the surface displacement shown in FIG. 11A is termedAmplitude₁. FIG. 11B depicts the horn surface 1100 of FIG. 11A, as itappears when stimulated by a voltage signal having an amplitude ofAmplitude₂ (Amplitude₂ is less than Amplitude₁). As can be seen fromcomparison between FIGS. 11A and 11B, the gap between the surface of thehorn 1100 and the anvil 1106 grows when the amplitude of the voltagesignal stimulating the horn diminishes, because the surface of the horn1100 is not so greatly displaced toward the anvil.

As mentioned previously, during a typical welding operation, a horn mayexhibit a surface displacement on the order of 3 mils, for example.However, the welding operation may yield satisfactory product, even ifthe surface displacement is reduced by, for example, 33%. Thus, per theaforementioned example, the welding operation may be performed with thehorn exhibiting a displacement of as little as 2 mils. It follows, then,that the welding operation may be initiated using an electrical signalof sufficient amplitude to stimulate a surface displacement of 3 mils.During operation, the horn experiences thermal expansion, meaning thatthe gap between the horn and the anvil diminishes as the horn expandstowards the anvil. To counteract this effect, the amplitude of theelectrical signal stimulating the horn may be attenuated, so as to yielda surface displacement less than the original 3 mils, therebymaintaining a substantially constant gap. Of course, in the context ofan operation that requires at least 2 mils of displacement to produce anappropriate product, the electrical signal should not be attenuated tosuch an extent that the surface of the horn exhibits less than therequired 2 mils of displacement.

An exemplary embodiment of a system for controlling the gap between ahorn and an anvil is depicted in FIG. 12A. As can be seen from FIG. 12A,the system includes a horn 1200 (which, in turn, includes the converterand booster), which is supplied with an AC electrical signal from apower supply 1202. The power supply 1202 communicates the resonantfrequency of the horn 1200 to a gap determining module 1204. (Asdescribed previously, the power supply 1202 detects the resonantfrequency of the horn stack and drives the horn stack at thatfrequency.)

The gap determining module 1204 determines the length of the gap (or,may determine the change in the gap, or may determine any other valuestanding in known relation to the length of the horn), based upon theresonant frequency, as described previously. Thereafter, the gap length(or change therein) is supplied to an amplitude determining module 1206.In response, the amplitude determining module identifies the properamplitude of the electrical signal to be delivered by the power supply,in order to maintain the gap substantially constant. The amplitude maybe retrieved from a look-up table, or may be arrived at by calculation.The amplitude determined thereby is communicated to a control signalgeneration module 1208, which generates an appropriate command orcontrol signal to cause the power supply 1202 to adjust the amplitude ofthe signal to that selected by the amplitude determination module 1206.

As described previously, each of the modules 1204-1208 may be embodiedas dedicated hardware, such as one or more ASICs cooperating with oneanother. Alternatively, the modules 1204-1208 may be embodied assoftware/firmware stored in a memory, and executed by a processor incommunication therewith. If embodied as firmware/software, theinstructions making up the modules 1204-1208 may be executed by the sameprocessor, or may be executed by a plurality of processors, as a matterof design choice.

Another exemplary embodiment of a system for controlling the gap betweena horn and an anvil is depicted in FIG. 12B. The system of FIG. 12Btakes advantage of two different schemes by which the gap may beadjusted: (1) controlling the position of the horn, itself; and (2)controlling the amount of surface displacement exhibited by the horn. Ascan be seen from FIG. 12B, the system includes a horn 1210 (which, inturn, includes the converter and booster), which is supplied with an ACelectrical signal from a power supply 1212. The power supply 1212communicates the resonant frequency of the horn 1210 to a gapdetermining module 1214. (As described previously, the power supply 1212detects the resonant frequency of the horn stack and drives the hornstack at that frequency.)

The gap determining module 1214 determines the length of the gap (or,may determine the change in the gap, or may determine any other valuestanding in known relation to the length of the horn), based upon theresonant frequency, as described previously. Thereafter, the gap length(or change therein) is supplied to an amplitude determining module 1216and to an adjustor 1220. The adjustor 1220 is a system that can alterthe position of the horn, such as the adjusting systems shown in FIGS. 7and 10, which adjust the position of the horn by varying the deformationof an elastic stop by varying degrees. As was the case in the embodimentof FIG. 12A, the amplitude determining module 1216 identifies the properamplitude of the electrical signal to be delivered by the power supply,in order to maintain the gap substantially constant. However, theamplitude determining unit 1216 cooperates with the adjustor 1220 tojointly adjust the position and/or adjust the amplitude of the AC signaldelivered by the power supply 1212, in order to achieve the end goal ofsubstantially maintaining a constant gap.

For example, according to one embodiment, the amplitude determinationunit 1216 and adjustor 1220 operate according to the method depicted inFIG. 13. As shown therein, both modules 1216 and 1220 receive the gaplength, or change therein, from the gap determining unit 1214, as shownin operation 1300. Thereafter, (assuming the embodiment in which theadjustor 1220 comprises a force applicator that forces the horn againsta deformable elastic stop), the amplitude determination unit 1216receives from the adjustor 1220 the force applied thereby (operation1302). Next, as shown in operation 1304, the force is compared to thelower limit of the acceptable force for the welding operation. If theforce is still above the limit, then the adjustor 1220 determines thenew force required for application, and adjusts the force accordingly(operation 1306). On the other hand, if the force has reached the lowerlimit, then the force should not be reduced any further, and control ispassed to operation 1308, in which it is determined whether theamplitude of the surface displacement has reached its lower limit. Ifnot, control is passed to operation 1310, whereupon the amplitudedetermining module 1216 identifies the proper amplitude of theelectrical signal to be delivered by the power supply, in order tomaintain the gap substantially constant. The amplitude determinedthereby is communicated to a control signal generation module 1218,which generates an appropriate command or control signal to cause thepower supply 1212 to adjust the amplitude of the signal to that selectedby the amplitude determination module 1216. On the other hand, if theamplitude of the surface displacement has reached its lower limit, thencontrol is passed to operation 1312, and an alarm is generated toindicate that the gap cannot be maintained at a constant length withouteither reducing the process force beneath its acceptable limit, orreducing the surface displacement of the horn beneath its acceptablelimit.

Although the operations of FIG. 13 are described as being performed byamplitude determination module 1216, the operations may be performed byany of the modules depicted in FIG. 12B, or may be performed by anothermodule dedicated to coordinating the operations of the amplitudedetermination module 1216 and the adjustor 1220.

Further, it is to be noted that, in operation 1302, the adjustor 1220may communicate the position of the horn to the module performing themethod of FIG. 13. Then, in operation 1304, the position of the horn maybe compared to a positional limit expressing the capacity of theadjustor 1220 to withdraw the horn from the anvil. In other words, inoperation 1304, it is determined whether the adjustor 1220 has withdrawnthe horn from the anvil as the adjust 1220 is able to do so.

According to another embodiment, the amplitude determination unit 1216and adjustor 1220 operate according to the method depicted in FIG. 14.As shown therein, both modules 1216 and 1220 receive the gap length, orchange therein, from the gap determining unit 1214, as shown inoperation 1400. Thereafter, (again assuming the embodiment in which theadjustor 1220 comprises a force applicator that forces the horn againsta deformable elastic stop), the amplitude determination unit 1216receives from the adjustor 1220 the force applied thereby (operation1402). Next, as shown in operation 1404, whereupon it is determinedwhether the amplitude of the surface displacement has reached its lowerlimit. If not, control is passed to operation 1406, whereupon theamplitude determining module 1216 identifies the proper amplitude of theelectrical signal to be delivered by the power supply 1212, in order tomaintain the gap substantially constant. The amplitude determinedthereby is communicated to a control signal generation module 1218,which generates an appropriate command or control signal to cause thepower supply 1212 to adjust the amplitude of the signal to that selectedby the amplitude determination module 1216. On the other hand, if theamplitude of the surface displacement exhibited by the horn has reachedthe lower limit, then the force should not be reduced any further, andcontrol is passed to operation 1408, in which it is determined whetherthe force value received during operation 1402 is at the lower limit ofthe acceptable force for the welding operation. If the force is stillabove the limit, then the adjustor 1220 determines the new forcerequired for application, and adjusts the force accordingly (operation1410). On the other hand, if the force has reached the lower limit, thencontrol is passed to operation 1412, and an alarm is generated toindicate that the gap cannot be maintained at a constant length withouteither reducing the process force beneath its acceptable limit, orreducing the surface displacement of the horn beneath its acceptablelimit.

Although the operations of FIG. 14 are described as being performed byamplitude determination module 1216, the operations may be performed byany of the modules depicted in FIG. 12B, or may be performed by anothermodule dedicated to coordinating the operations of the amplitudedetermination module 1216 and the adjustor 1220.

Further, it is to be noted that, in operation 1402, the adjustor 1220may communicate the position of the horn to the module performing themethod of FIG. 14. Then, in operation 1408, the position of the horn maybe compared to a positional limit expressing the capacity of theadjustor 1220 to withdraw the horn from the anvil. In other words, inoperation 1408, it is determined whether the adjustor 1220 has withdrawnthe horn from the anvil as the adjust 1220 is able to do so.

Upon reading and understanding the foregoing process for controlling anultrasonic welding system, one of ordinary skill in the art willappreciate that gap control for a system can be achieved by measuringthe operating frequency of the horn, and then adjusting the force, forexample, pressure, that controls the gap. The specific equations can bederived or determined empirically for any horn geometry, includinglinear and rotary horns.

The various embodiments described above are provided by way ofillustration only and should not be construed to limit the invention.Those skilled in the art will readily recognize various modificationsand changes that may be made to the present invention without followingthe example embodiments and applications illustrated and describedherein, and without departing from the true spirit and scope of thepresent invention, which is set forth in the following claims.

1. A system for applying ultrasonic energy to a workpiece, the systemcomprising: a horn stack; a mounting system upon which the horn stack ismounted; a source of energy coupled to the horn stack; an anvil having asurface for supporting the workpiece; and a controller configured toreceive a resonant frequency of the horn stack, and to determine aquantity standing in known relation to a change in gap between the hornstack and the anvil.
 2. The system of claim 1, wherein the horn stackcomprises: a converter, a booster, and a horn.
 3. The system of claim 1,wherein the mounting system mates with the horn stack substantially at anodal point of the horn stack.
 4. The system of claim 1, wherein thehorn stack has a longitudinal axis about which the horn rotates, andwherein the longitudinal axis is substantially parallel to thesupporting surface of the anvil.
 5. The system of claim 1, wherein thehorn stack has a longitudinal axis, and wherein the longitudinal axis issubstantially perpendicular to the supporting surface of the anvil. 6.The system of claim 1, further comprising a cooling system for coolingthe converter and booster during operation of the system.
 7. The systemof claim 6, wherein the cooling system prevents significant thermalexpansion of the booster and converter during operation of the system.8. The system of claim 1, further comprising: a position adjustmentsystem coupled to the horn stack, for adjusting the position of the hornstacks, so as to maintain a substantially constant gap between the hornstack and the anvil during operation of the system.
 9. The system ofclaim 1, further comprising: a position adjustment system coupled to thehorn stack, for adjusting the position of the horn stacks, based uponthe resonant frequency of the horn stack.
 10. The system of claim 1,further comprising: a position adjustment system coupled to the hornstack, for adjusting the position of the horn stacks, based upon acontrol signal generated from the resonant frequency of the horn stack.11. A system for applying ultrasonic energy to a workpiece, the systemcomprising: a horn stack; a mounting system upon which the horn stack ismounted; a source of energy coupled to the horn stack; an anvil having asurface for supporting the workpiece; and a means for determining aquantity standing in known relation to a change in gap between the hornstack and the anvil.